A new model tries to explain why you’re tired after a long day at work

You leave the office after a long day of work that has been particularly exhausting from a cognitive point of view. You arrive home and you hesitate between cooking and ordering a pizza. Eventually, you go for the pizza because you feel too tired. A recent experiment carried out by French researchers from the Institut du Brain in Paris, published in Current Biologybrings food for thought to what is probably going on in your brain at that time.

What might be going on in your brain

We can only be speculative at this time given the non-interventional nature of this study. It merely observes the links between several variables (cognitive fatigue, economic choices, activation of certain brain areas, the presence of certain metabolites, etc.). But according to the theoretical model of the researchers, here is what happens in your brain although this remains to be demonstrated. A cerebral system would be responsible for exercising a permanent meta-control between the expected benefits of performing certain executive functions and the cost from a metabolic point of view. From this balance would result the activation (or not) of the cerebral areas associated with cognitive control which would determine the final behavior: to engage again in a costly task or to opt for an inexpensive option. When cognitive fatigue sets in (or simply when we are drawn to less expensive activities without even feeling this fatigue), it is because the level of glutamate is no longer optimal (this point will be detailed below, but according to the study, it remains at too high a level for too long during expensive tasks). Consequently, a metabolic regulation loop attenuates the activation of the cerebral areas which govern the control of the executive functions and this decrease in activation results in the cessation of the task towards a less costly behavior.

Some basics to better understand the details of the study

Neuroscience makes a good distinction between automated tasks and those that require cognitive effort. For example, professional chess players can make the first moves of a game (this is called the opening) very quickly, but when the position becomes more complex and the possibilities of moves become almost endless, they must reflect and adapt their computational skills to the particular position they have under the eyes.

In research, tasks have been developed by scientists to study executive functions (including cognitive control) and resulting cognitive fatigue in the laboratory. Generally, studies operate within the theoretical framework of the association between a stimulus and its response. Two main paradigmatic tasks exist: N-back and the N-switch. The N-back mainly involve the working memory by asking participants to recall certain information between a stimulus and a response they were given earlier. The N-switchon the other hand, target more theinhibition cognitive – that is, when we voluntarily prevent automatic behavior to adapt to the situation – by varying the stimulus-response association, so that the appropriate response is not always that associated with the memories present in Memory of work. These tasks have been used to determine which regions of the brain are activated when we use our executive functions. We now know that the majority is located at the prefrontal, lateral and parietal level of the cortex. However, it is not yet clear why this causes cognitive fatigue.

Hypotheses have been proposed at several levels. From a biochemical point of view, some believe that such tasks consume more glucose. The hypotheses formulated around this idea are grouped together under the name of resource depletion theory. However, these hypotheses lack empirical, robust and reliable evidence, and do not explain the specificity of executive functions in the occurrence of cognitive fatigue compared to another process such as vision for example.

From an economic point of view, fatigue would in fact be induced directly by the brain itself in order to suggest that we stop the current task and direct us to an activity that would induce a reward more quickly with a lower cost. In this theoretical framework, fatigue comes from a cost-reward calculation that helps to adjust our behavior in order to choose the most attractive opportunity. If arguments support this hypothesis such as the fact that increasing the level of reward makes it possible to persevere in a costly task despite fatigue, other arguments make this view problematic.

Indeed, cognitive fatigue is not something that necessarily appears. For example, you may be doing an expensive task and stop because it’s lunch time, but still not be tired. You just do it because there is a good reason to stop the current activity. An even more salient argument is the fact that most pathological conditions that impair executive functions and generate chronic cognitive fatigue, such as burnout or depression, hinders the planning of actions to seek a reward in sick people. Indeed, these diseases are often characterized by the desire to do nothing, not even to take pleasure. If no single theory does the trick, perhaps they need to be put together. This is what the authors of the study propose.

The theoretical framework and some preliminary results

For their study, the authors make two assumptions. First, cognitive fatigue results from an increase in the cost of exerting cognitive control within a task. Secondly, this increase translates into metabolic alteration within the cerebral systems guaranteeing cognitive control (at the level of prefrontal cortex, lateral and parietal, therefore). To assess the cognitive fatigue, the authors have chosen to use economic games. They think that the drop in performance during a timed task can be explained by boredom, can be counteracted by training or compensated by motivation. They therefore consider that the systematic choice of an inexpensive option (a choice bias for low-cost) represents an objective marker of cognitive fatigue, even in the absence of a feeling of fatigue by the individual. Investigators are using an experimental design developed for an earlier study. Two groups must perform the same task, only the level of difficulty changes. Also, they will carry out their measurements on a region of the brain not associated with cognitive control (the visual cortex) in order to have comparable data. To standardize this measure, the participants were subjected to pre-tests beforehand to determine their respective individual level concerning the cost-benefit balance, so that the dilemmas were adapted and were not too easy for some and too difficult for others. the others, which would have come to encroach on the robustness of the study.

In the old study in question, they have already highlighted that the difficult level results in a tendency to choose the immediate reward rather than a distant reward within an economic game. Through a technique ofMagnetic resonance imaging, they were also able to show that the choices following difficult tasks were associated with less recruitment of the areas of the brain in charge of cognitive control. Their first supposition therefore seems valid experimentally speaking: cognitive fatigue seems to occur when the cost of an energy-intensive cognitive task is too high. To assess their second guess, they used magnetic resonance spectroscopy, which quantifies metabolites within the neuronal tissues in question. They particularly focused on the detection of glutamate and its metabolites, which represents an ideal candidate for their research. Indeed, this neurotransmitter may be responsible for the alteration of the functioning of the neuron or communications between neurons when it is not present in optimal quantity (lack or excess).

The experience in question

To induce cognitive fatigue in participants, the researchers used the two experimental paradigms mentioned above. For more than six hours, 16 of them performed the “easy” tasks and 24 the “difficult” tasks. The first consisted in distinguishing between vowel and consummate or uppercase and lowercase letters by identifying the associations of colors associated with the different types of letter (green or red). In the easy version, the color changed only once during the 24 repetitions of the task within a session while in the difficult version, it changed twelve times. The second differed in purpose. The same letters were displayed but the participants had to memorize them in order to answer the following question: are the letters displayed the same or different than the previous task (easy version) / than an old task further away (the one that preceded the current task of two trials). At the end of each block of 24 successive tasks, participants “played” an economic game where there were two options to choose from: one conferred a large reward but with a cost (waiting time, uncertainty, effort physicalcognitive effort), while the other conferred a more modest reward without requiring effort.

What the researchers observe is consistent with the integrative hypothesis (metabolic accumulation + functional role) of their model – except that instead of observing an accumulation of glutamate during the successive performance of difficult tasks, they observe a drop in the latter when performing easy tasks. As noted above, these data are correlational and do not represent evidence of a causal mechanism. However, they provide avenues of research that could be used by future studies to confirm or refute the links observed.